Field of the Invention
The present invention relates to an ink jet print head having an actuator which consists of a longitudinal vibration mode piezoelectric vibrator.
Related Art
A conventional on-demand type ink-jet recording head comprises a plurality of pressure generating chambers for generating ink pressure by means of piezo-electric vibrators and heating elements. A common ink chamber supplies ink to each of the pressure generating chambers via a flow channel for each respective pressure generating chamber. Each pressure generating chamber communicates with a nozzle so that the ink-jet recording head can send a stream of ink drops from each nozzle to a recording medium in accordance with a driving signal to the pressure generating chambers. The driving signal corresponds to a print signal.
In a first conventional ink-jet recording head, a resistive wire for generating Joule heat is provided in the pressure generating chamber as the pressure generating means for causing ink drops to be discharged through the nozzle. This conventional device makes use of bubble generating pressure and is known as a bubble-jet type print device.
On the other hand, a high speed drive actuator for an ink jet print head expands and compresses a pressure generating chamber to suck in ink and to form ink droplets. The actuator is constructed with a piezoelectric vibrator having a longitudinal vibration mode, which is expandable in its axial direction and has a structure consisting of piezoelectric sheet-like members and conductive sheet-like members, alternately layered one on another. A part of the pressure generating chamber is formed with an elastic plate, and the chamber communicates with a nozzle hole associated therewith.
Although the bubble-jet type head makes it feasible to readily provide an inexpensive, high-density apparatus, the heat generation causes the deterioration of ink and the head itself. By contrast, the piezoelectric type features no ink deterioration because heat generation is not a factor. Therefore, a wide range of inks may be used, and lower operating costs result because the life of the head is semipermanent. Moreover, the high-speed driving of the piezo-electric vibrator having the vertical vibration mode, and the alternate repetition of the expansion and contraction of the pressure generating chamber by bringing the piezo-electric vibrator into contact with the pressure generating chamber allows the piezo-electric type print head provide higher speed printing than the bubble-jet type.
Further, when the longitudinal vibration mode piezoelectric vibrator is compared with a piezoelectric vibrator of the type in which the surface thereof is bent for vibration, the former has a smaller contact area where it contacts with the pressure generating chamber than the latter, and may be driven at higher speed than the latter. Accordingly, the former is capable of performing the printing operation at a higher speed and also at higher resolution. Therefore, while both types of piezo-electric vibration modes may be used, the longitudinal type is preferable.
While the longitudinal vibration mode piezoelectric vibrator may be driven at high speed, the attenuation rate of the residual vibration is small. This is because fluctuations in pressure remain in the chamber even after the pressure is generated in the chamber to discharge ink drops. After discharge, an ink meniscus within the nozzle recovers toward the tip of the nozzle at a resonance period (Helmholtz resonance period) specific to the materials and dimensions of pressure generating chamber. As a result, a large vibration is left after an ink droplet is shot forth.
Because the residual vibration affects the behavior of the meniscus, the position of the meniscus is indefinite when the next ink droplet is to be jet out. This may be explained by the fact that the period of the residual vibration is minute and shorter than the time required for the meniscus to reach the tip of the nozzle (the time is hereinafter called the xe2x80x9crecovery timexe2x80x9d of the meniscus). When high-frequency driving is carried out, the discharged ink drops may become unstable because the meniscus is insecure if ink drops are caused to be discharged before the minute residual vibration is sufficiently settled. Consequently, the direction in which the ink drops are jetted from the nozzle varies, and ink misting occurs when the meniscus overshoots the nozzle. The result is deterioration of the print quality. This hampers improvements in the response frequency of the ink jet recording head.
The vibration behavior of the meniscus varies not only with dimensional variations in the flow channel but also varies with the physical properties of material and ink. The environmental temperature makes the meniscus behavior vary further. Thus, the residual vibration of the meniscus cannot effectively controlled by a fixed driving method. Because many variables must be considered, the production cost increases. In addition, freedom in design is reduced because the dimensions of the flow channel need severe control, and less latitude is allowed in selecting material for use in forming the flow channel and for ink selection.
In addition, there arises the following problems. When the pressure generating chamber is expanded, the meniscus within the nozzle is drawn to the pressure generating chamber side. However, the meniscus is gradually recovered toward the tip of the nozzle as ink is gradually supplied into the pressure generating chamber. The discharge speed of ink drops is made constant by causing ink to be discharged after the meniscus reaches the tip of the nozzle, irrespective of the discharge timing. When the high-frequency driving is carried out, however, the ink has to be discharged before the meniscus thus drawn satisfactorily reaches the tip of the nozzle, depending on the recovery time of the meniscus since the expansion and contraction of the pressure chamber need to be carried out at short lead time.
Moreover, it is preferred to have the ink discharged in such a state that the meniscus has been drawn in to a certain degree in order to secure the discharge speed of ink drops and a stable discharge of ink.
The drawn quantity of the meniscus and the recovery time up to the tip of the nozzle vary with the dimensions of the flow channel and the physical properties of material and ink, similar to the meniscus vibration after the ink is discharged. Consequently, the method of causing ink to be discharged at fixed timing produces variation in the drawing position of the meniscus at the time of discharging ink. This varies the discharge speed of ink drops and the discharge quantity of ink. As set forth above, to maintain consistent print quality taking into account these factors, the production cost increases, whereas freedom of design is reduced because the dimensions of the flow channel need severe control, and less latitude is allowed in selecting material for use in forming the flow channel and also for ink selection.
The present invention overcomes the problems noted above. An object of the present invention is to provide an ink jet printing device which is driven at high speed while being free from the generation of ink mist and the bending of the flying path of the ink droplet. Such a printing device offers stable images even at high drawing frequencies by maintaining a constant ink discharge speed. This ensures consistent positioning of the ink spots.
A second object of the present invention is to provide an ink jet printing device which is capable of changing dot size while maintaining print quality.
A third object of the present invention is to provide an ink jet printing device which is driven at a preset drive frequency independently of the specifications of the print head and ambient temperature, and which is free from the generation of ink mist and the bending of a flying path of the ink droplet.
A fourth object of the present invention is to provide an ink jet printing device which is driven according to dimensions of the ink flow channels, physical properties of the material and ink, and environmental temperature.
To solve the problems referred to above, the present invention comprises: an ink jet print head having pressure generating chambers each including a nozzle hole and each communicating with a common ink chamber, the pressure generating chambers each having a Helmholtz resonance frequency of period TH and communicating through an ink supplying path, and a piezoelectric vibrator for expanding and compressing said pressure generating chambers; and drive signal generating means for generating a first signal to expand said pressure generating chambers, a second signal to compress said pressure generating chambers being in an expanded state to compel said pressure generating chamber to shoot forth an ink droplet through said nozzle hole, and a third signal to expand said pressure generating chambers by a volume smaller than the volume expanded by said first signal when the vibration of the meniscus generated after the shooting of an ink droplet moves to the nozzle hole. The first through third signals may be in the form of pulses.
In another embodiment of the invention, the printing device further includes a drive signal generating control means to selectively control the timing for the start of the second and third pulses.
The timing of the start of the second signal is controlled by the drive signal generating control means so that the position of a meniscus in the nozzle at the timing of starting the second pulse is made constant. The drive signal generating control means sets the timing of the start of the second signal as desired according to the flow-channel impedance of the nozzle and the ink supply port. The timing of the start of the second signal is set fast when the flow-channel impedance of the nozzle or the ink supply port is low, whereas the timing thereof is set slow when the flow-channel impedance thereof is high. The timing of the start of the second signal is set fast when the sectional area of the nozzle or the ink supply port is large, whereas the timing thereof is set slow when the sectional area thereof is small. The timing of the start of the second signal is set fast when the nozzle or the ink supply port is long, whereas the timing thereof is set slow when the nozzle or the ink supply port is short.
The ink-jet recording apparatus further comprises an environmental temperature detection means, so that the timing of the start of the second signal is controlled by the drive signal generating control means according to the environmental temperature. The timing of the start of the second signal is set fast when the environmental temperature rises, whereas the timing thereof is set slow when the environmental temperature lowers.
When the vibration of the meniscus generated by the shooting of an ink droplet moves toward the nozzle hole, the pressure generating chamber receives the third signal to minutely expand the pressure generating chamber to effectively attenuate the vibration of the meniscus, and to stay the meniscus at a position suitable for jetting out the next ink droplet.
The timing of the start of the third signal is controlled by the drive signal generating control means so that the vibration of the meniscus generated after the ink drops are discharged substantially conforms to the vibration thereof at a point of time the meniscus is moved closest to the pressure generating chamber.
The ink-jet recording apparatus further comprises the drive signal generating control means for selectively setting the timing of starting the third signal according to the Helmholtz period TH of the pressure generating chamber. The duration of the second signal is set substantially equal to the duration of the third signal and the time from the start of the second signal up to the start of the third signal is set to substantially conform to the Helmholtz period TH of the pressure generating chamber.